crypto/x509: add ExtKeyUsage.String and KeyUsage.String methods

crypto/x509: add ExtKeyUsage.String and KeyUsage.String methods

Fixes #56866

Change-Id: Icc8f067820f5d74e0d5073bce160429e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/723360
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Roland Shoemaker <roland@golang.org>